F9 and hemophilia B: To extend the circulating half-life of FIX and, therefore, reduce the dosing frequency for hemophilia B therapy, we have developed a recombinant factor IX Fc fusion protein (rFIXFc) that is composed of a single molecule of rFIX covalently fused to the Fc domain of human immunoglobulin G1 (IgG1) with no intervening linker sequence [8,9].